1. Field of the Invention
This invention relates generally to the field of shock protection for fragile quartz crystals and the like. More particularly, this invention relates to a shock absorbing mechanism and spacer for protecting quartz crystals from impact with the walls of its enclosure and secondary impact against surrounding components.
2. Background
Quartz crystals are frequently used in electronic radio equipment such as two-way radios, pagers and the like as high stability frequency determining elements. They are also used in such radio equipment as filtering elements due to their very high "Q" and therefore high selectivity.
Unfortunately, it has long been recognized that such crystal devices are usually the most fragile components in the radio equipment. This presents special problems when the radio equipment is used in an environment which makes it especially susceptible to high mechanical shock such as police radios or pagers which may be subject to frequent drops. In these environments it is not unusual for crystal devices to shatter or crack when presented with excess mechanical shock. The problem is compounded by the rapid miniaturization of such equipment making it subject to higher impact velocities when carelessly tossed about.
In order to enhance the reliability of such electronic equipment, it is clearly necessary to provide better mechanical shock protection for such crystal devices. Crystal devices which can consistently withstand shocks of approximately 20,000 to 30,000 times the force of gravity (20,000 to 30,000 G's) and greater for approximately 0.3 milliseconds are needed to insure the reliability of such electronic devices at present and in the future even greater shock performance will be necessary. At present, shocks in excess of this limit are absorbed by deformation and damage to the plastic enclosures typically used on such equipment.
A number of solutions to this problem have been proposed and have met with varying degrees of success. Unfortunately, none of these proposals have been able to reliably and consistently enable such crystal devices to withstand mechanical shocks in excess of approximately 15,000 G's. Such proposals have included simply coating the inner surface of a crystal enclosure with plastic to absorb shock and inserting short sections of plastic tubing or plastic strips inside the crystal enclosure to absorb shock. While such techniques provide some degree of improvement in the shock levels crystal devices can withstand without them (typically approximately 2000 to 3000 G's unprotected), further improvement is required to achieve acceptable levels of reliability.
In order to further increase the crystal devices ability to survive mechanical shock, thick foam or plastic shock cushions are often placed over the exterior of the crystal enclosure to help absorb the shock of impact with surrounding components. These provide some degree of protection by absorbing a portion of the shock with the compressive spring properties of the material itself, but are space consuming and only partially effective since the spring rate is only controllable by virtue of selection of type and thickness of material.
Further enhancement may be obtained by spacing the crystal device above its circuit board with a resiliently spacer to enable the device's electrical leads to absorb a portion of the shock. This technique has the added advantage of reducing the incidence of leads actually being pulled from the crystal device. Unfortunately, use of exterior shock cushions in conjunction with spacers greatly increases the handling cost of inserting the crystal device into the circuit board and is collectively very space inefficient.